Method for transmitting information for lte-wlan aggregation system and a device therefor

ABSTRACT

The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting information for LTE-WLAN aggregation system, the method comprising: checking, by a Packet Data Convergence Protocol (PDCP) entity, whether a FCI transmission condition is met or not; and transmitting Flow Control Information (FCI) when the FCI transmission condition is met.

TECHNICAL FIELD

The present invention relates to a wireless communication system and, more particularly, to a method for transmitting information for LTE-WLAN aggregation system and a device therefor.

BACKGROUND ART

As an example of a mobile communication system to which the present invention is applicable, a 3rd Generation Partnership Project Long Term Evolution (hereinafter, referred to as LTE) communication system is described in brief.

FIG. 1 is a view schematically illustrating a network structure of an E-UMTS as an exemplary radio communication system. An Evolved Universal Mobile Telecommunications System (E-UMTS) is an advanced version of a conventional Universal Mobile Telecommunications System (UMTS) and basic standardization thereof is currently underway in the 3GPP. E-UMTS may be generally referred to as a Long Term Evolution (LTE) system. For details of the technical specifications of the UMTS and E-UMTS, reference can be made to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs (eNBs), and an Access Gateway (AG) which is located at an end of the network (E-UTRAN) and connected to an external network. The eNBs may simultaneously transmit multiple data streams for a broadcast service, a multicast service, and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in one of bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides a downlink (DL) or uplink (UL) transmission service to a plurality of UEs in the bandwidth. Different cells may be set to provide different bandwidths. The eNB controls data transmission or reception to and from a plurality of UEs. The eNB transmits DL scheduling information of DL data to a corresponding UE so as to inform the UE of a time/frequency domain in which the DL data is supposed to be transmitted, coding, a data size, and hybrid automatic repeat and request (HARQ)-related information. In addition, the eNB transmits UL scheduling information of UL data to a corresponding UE so as to inform the UE of a time/frequency domain which may be used by the UE, coding, a data size, and HARQrelated information. An interface for transmitting user traffic or control traffic may be used between eNBs. A core network (CN) may include the AG and a network node or the like for user registration of UEs. The AG manages the mobility of a UE on a tracking area (TA) basis. One TA includes a plurality of cells.

Meanwhile, various wireless communication technologies systems have been developed with rapid development of information communication technologies. WLAN technology from among wireless communication technologies allows wireless Internet access at home or in enterprises or at a specific service provision region using mobile terminals, such as a Personal Digital Assistant (PDA), a laptop computer, a Portable Multimedia Player (PMP), etc. on the basis of Radio Frequency (RF) technology.

A standard for a wireless LAN technology is developing as IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard. IEEE 802.11a and b use an unlicensed band on 2.4 GHz or 5 GHz. IEEE 802.11b provides transmission speed of 11 Mbps and IEEE 802.11a provides transmission speed of 54 Mbps. IEEE 802.11g provides transmission speed of 54 Mbps in a manner of applying an OFDM (orthogonal frequency-division multiplexing) scheme on 2.4 GHz. IEEE 802.11n provides transmission speed of 300 Mbps to 4 spatial streams in a manner of applying a MIMO-OFDM (multiple input multiple output-OFDM) scheme. IEEE 802.11n supports a channel bandwidth as wide as 40 MHz. In this case, it is able to provide transmission speed of 600 Mbps.

The aforementioned wireless LAN standard has been continuously enhanced and standardization of IEEE 802.11ax, which is appearing after IEEE 802.11ac standard supporting maximum 1 Gbps by using maximum 160 MHz channel bandwidth and supporting 8 spatial streams, is under discussion.

Recently, a radio technology has been developed in two types in response to the rapid increase of traffic. Firstly, speed of the radio technology itself is getting faster. A mobile phone wireless internet technology has been developed from HSPA to LTE and LTE to LTE-A. Currently, the mobile phone wireless internet technology becomes fast as fast as maximum 225 Mbps and a Wi-Fi technology becomes fast as fast as maximum 6.7 Gbps in case of IEEE 802.11 ad. Secondly, speed can be increased using a scheme of aggregating a plurality of radio channels with each other. For example, there exists LTE-A which supports carrier aggregation corresponding to a technology of binding frequency bands using an identical radio technology into one. In this context, necessity for a technology of aggregating heterogeneous wireless internet is emerging. It is necessary to develop a scheme of transmitting data by biding radio technologies (e.g., LTE and wireless-LAN) including characteristics different from each other.

DISCLOSURE OF INVENTION Technical Problem

An object of the present invention devised to solve the problem lies in a method and device for transmitting information for WLAN network for LTE-WLAN aggregation system. The technical problems solved by the present invention are not limited to the above technical problems and those skilled in the art may understand other technical problems from the following description.

Solution to Problem

The object of the present invention can be achieved by providing a method for User Equipment (UE) operating in a wireless communication system as set forth in the appended claims.

In another aspect of the present invention, provided herein is a communication apparatus as set forth in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

Advantageous Effects of Invention

According to the present invention, the PDCP receiver transmits a “Flow Control Information (FCI)” to the PDCP transmitter under a certain condition in order to reduce a signaling overhead caused by frequent transmission of PDCP status report while providing necessary information for a flow control in a timely manner.

It will be appreciated by persons skilled in the art that the effects achieved by the present invention are not limited to what has been particularly described hereinabove and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention.

FIG. 1 is a diagram showing a network structure of an Evolved Universal Mobile Telecommunications System (E-UMTS) as an example of a wireless communication system;

FIG. 2A is a block diagram illustrating network structure of an evolved universal mobile telecommunication system (E-UMTS), and FIG. 2B is a block diagram depicting architecture of a typical E-UTRAN and a typical EPC;

FIG. 3 is a diagram showing a control plane and a user plane of a radio interface protocol between a UE and an E-UTRAN based on a 3rd generation partnership project (3GPP) radio access network standard;

FIG. 4 is a view showing an example of a physical channel structure used in an E-UMTS system;

FIGS. 5 to 7 illustrate are exemplary configuration of an IEEE 802.11 system to which the present invention is applicable;

FIG. 8 illustrates an exemplary configuration of a WLAN system;

FIG. 9 is a block diagram of a communication apparatus according to an embodiment of the present invention;

FIG. 10 is a diagram for a LTE-WLAN aggregation (LWA) Radio Protocol Architecture according to embodiments of the present invention;

FIG. 11 is an exemplary for a LTE-WLAN aggregation architecture;

FIG. 12 is an example for Dual Connectivity 3C-like approaches using GTP-U;

FIG. 13 is a diagram for PDCP control PDU for PDCP status report;

FIG. 14 is a diagram for transmitting Flow Control Information for a LTE-WLAN aggregation (LWA) according to embodiments of the present invention; and

FIG. 15 is an example for transmitting Flow Control Information for a LTE-WLAN aggregation (LWA) according to embodiments of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Universal mobile telecommunications system (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.

Hereinafter, structures, operations, and other features of the present invention will be readily understood from the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Embodiments described later are examples in which technical features of the present invention are applied to a 3GPP system.

Although the embodiments of the present invention are described using a long term evolution (LTE) system and a LTE-advanced (LTE-A) system in the present specification, they are purely exemplary. Therefore, the embodiments of the present invention are applicable to any other communication system corresponding to the above definition. In addition, although the embodiments of the present invention are described based on a frequency division duplex (FDD) scheme in the present specification, the embodiments of the present invention may be easily modified and applied to a half-duplex FDD (H-FDD) scheme or a time division duplex (TDD) scheme.

FIG. 2A is a block diagram illustrating network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed to provide a variety of communication services such as voice (VoIP) through IMS and packet data.

As illustrated in FIG. 2A, the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN), an Evolved Packet Core (EPC) and one or more user equipment. The E-UTRAN may include one or more evolved NodeB (eNodeB) 20, and a plurality of user equipment (UE) 10 may be located in one cell. One or more E-UTRAN mobility management entity (MME)/system architecture evolution (SAE) gateways 30 may be positioned at the end of the network and connected to an external network.

As used herein, “downlink” refers to communication from eNodeB 20 to UE 10, and “uplink” refers to communication from the UE to an eNodeB. UE 10 refers to communication equipment carried by a user and may be also referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.

FIG. 2B is a block diagram depicting architecture of a typical E-UTRAN and a typical EPC.

As illustrated in FIG. 2B, an eNodeB 20 provides end points of a user plane and a control plane to the UE 10. MME/SAE gateway 30 provides an end point of a session and mobility management function for UE 10. The eNodeB and MME/SAE gateway may be connected via an S1 interface.

The eNodeB 20 is generally a fixed station that communicates with a UE 10, and may also be referred to as a base station (BS) or an access point. One eNodeB 20 may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNodeBs 20.

The MME provides various functions including NAS signaling to eNodeBs 20, NAS signaling security, AS Security control, Inter CN node signaling for mobility between 3GPP access networks, Idle mode UE Reachability (including control and execution of paging retransmission), Tracking Area list management (for UE in idle and active mode), PDN GW and Serving GW selection, MME selection for handovers with MME change, SGSN selection for handovers to 2G or 3G 3GPP access networks, Roaming, Authentication, Bearer management functions including dedicated bearer establishment, Support for PWS (which includes ETWS and CMAS) message transmission. The SAE gateway host provides assorted functions including Per-user based packet filtering (by e.g. deep packet inspection), Lawful Interception, UE IP address allocation, Transport level packet marking in the downlink, UL and DL service level charging, gating and rate enforcement, DL rate enforcement based on APN-AMBR. For clarity MME/SAE gateway 30 will be referred to herein simply as a “gateway,” but it is understood that this entity includes both an MME and an SAE gateway.

A plurality of nodes may be connected between eNodeB 20 and gateway 30 via the S1 interface. The eNodeBs 20 may be connected to each other via an X2 interface and neighboring eNodeBs may have a meshed network structure that has the X2 interface.

As illustrated, eNodeB 20 may perform functions of selection for gateway 30, routing toward the gateway during a Radio Resource Control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of Broadcast Channel (BCCH) information, dynamic allocation of resources to UEs 10 in both uplink and downlink, configuration and provisioning of eNodeB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 may perform functions of paging origination, LTE-IDLE state management, ciphering of the user plane, System Architecture Evolution (SAE) bearer control, and ciphering and integrity protection of Non-Access Stratum (NAS) signaling.

The EPC includes a mobility management entity (MME), a serving-gateway (S-GW), and a packet data network-gateway (PDN-GW). The MME has information about connections and capabilities of UEs, mainly for use in managing the mobility of the UEs. The S-GW is a gateway having the E-UTRAN as an end point, and the PDN-GW is a gateway having a packet data network (PDN) as an end point.

FIG. 3 is a diagram showing a control plane and a user plane of a radio interface protocol between a UE and an E-UTRAN based on a 3GPP radio access network standard. The control plane refers to a path used for transmitting control messages used for managing a call between the UE and the E-UTRAN. The user plane refers to a path used for transmitting data generated in an application layer, e.g., voice data or Internet packet data.

A physical (PHY) layer of a first layer provides an information transfer service to a higher layer using a physical channel. The PHY layer is connected to a medium access control (MAC) layer located on the higher layer via a transport channel. Data is transported between the MAC layer and the PHY layer via the transport channel. Data is transported between a physical layer of a transmitting side and a physical layer of a receiving side via physical channels. The physical channels use time and frequency as radio resources. In detail, the physical channel is modulated using an orthogonal frequency division multiple access (OFDMA) scheme in downlink and is modulated using a single carrier frequency division multiple access (SC-FDMA) scheme in uplink.

The MAC layer of a second layer provides a service to a radio link control (RLC) layer of a higher layer via a logical channel. The RLC layer of the second layer supports reliable data transmission. A function of the RLC layer may be implemented by a functional block of the MAC layer. A packet data convergence protocol (PDCP) layer of the second layer performs a header compression function to reduce unnecessary control information for efficient transmission of an Internet protocol (IP) packet such as an IP version 4 (IPv4) packet or an IP version 6 (IPv6) packet in a radio interface having a relatively small bandwidth.

A radio resource control (RRC) layer located at the bottom of a third layer is defined only in the control plane. The RRC layer controls logical channels, transport channels, and physical channels in relation to configuration, re-configuration, and release of radio bearers (RBs). An RB refers to a service that the second layer provides for data transmission between the UE and the E-UTRAN. To this end, the RRC layer of the UE and the RRC layer of the E-UTRAN exchange RRC messages with each other.

One cell of the eNB is set to operate in one of bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides a downlink or uplink transmission service to a plurality of UEs in the bandwidth. Different cells may be set to provide different bandwidths.

Downlink transport channels for transmission of data from the E-UTRAN to the UE include a broadcast channel (BCH) for transmission of system information, a paging channel (PCH) for transmission of paging messages, and a downlink shared channel (SCH) for transmission of user traffic or control messages. Traffic or control messages of a downlink multicast or broadcast service may be transmitted through the downlink SCH and may also be transmitted through a separate downlink multicast channel (MCH).

Uplink transport channels for transmission of data from the UE to the E-UTRAN include a random access channel (RACH) for transmission of initial control messages and an uplink SCH for transmission of user traffic or control messages. Logical channels that are defined above the transport channels and mapped to the transport channels include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic channel (MTCH).

FIG. 4 is a view showing an example of a physical channel structure used in an E-UMTS system. A physical channel includes several subframes on a time axis and several subcarriers on a frequency axis. Here, one subframe includes a plurality of symbols on the time axis. One subframe includes a plurality of resource blocks and one resource block includes a plurality of symbols and a plurality of subcarriers. In addition, each subframe may use certain subcarriers of certain symbols (e.g., a first symbol) of a subframe for a physical downlink control channel (PDCCH), that is, an L1/L2 control channel. In FIG. 4, an L1/L2 control information transmission area (PDCCH) and a data area (PDSCH) are shown. In one embodiment, a radio frame of 10 ms is used and one radio frame includes 10 subframes. In addition, one subframe includes two consecutive slots. The length of one slot may be 0.5 ms. In addition, one subframe includes a plurality of OFDM symbols and a portion (e.g., a first symbol) of the plurality of OFDM symbols may be used for transmitting the L1/L2 control information. A transmission time interval (TTI) which is a unit time for transmitting data is 1 ms.

A base station and a UE mostly transmit/receive data via a PDSCH, which is a physical channel, using a DL-SCH which is a transmission channel, except a certain control signal or certain service data. Information indicating to which UE (one or a plurality of UEs) PDSCH data is transmitted and how the UE receive and decode PDSCH data is transmitted in a state of being included in the PDCCH.

For example, in one embodiment, a certain PDCCH is CRC-masked with a radio network temporary identity (RNTI) “A” and information about data is transmitted using a radio resource “B” (e.g., a frequency location) and transmission format information “C” (e.g., a transmission block size, modulation, coding information or the like) via a certain subframe. Then, one or more UEs located in a cell monitor the PDCCH using its RNTI information. And, a specific UE with RNTI “A” reads the PDCCH and then receive the PDSCH indicated by B and C in the PDCCH information.

FIG. 5 illustrates an exemplary configuration of an IEEE 802.11 system to which the present invention is applicable.

The IEEE 802.11 architecture may include a plurality of components. A WLAN that supports Station (STA) mobility transparent to upper layers may be provided through interaction between the components. A Basic Service Set (BSS) is a basic building block of an IEEE 802.11 LAN. FIG. 5 illustrates two BSSs, BSS1 and BSS2, each with two STAs that are members of the BSS (STA1 and STA2 are included in BSS1 and STA3 and STA4 are included in BSS2). Each of the BSSs covers an area in which the STAs of the BSS maintain communication, as indicated by an oval. This area may be referred to as a Basic Service Area (BSA). As an STA moves out of its BSA, it can no longer communicate directly with other members of the BSA.

An Independent Basic Service Set (IBSS) is the most basic type of BSS in the IEEE 802.11 LAN. For example, a minimum IBSS includes only two STAs. A BSS, BSS1 or BSS2 which is the most basic type without other components in FIG. 1 may be taken as a major example of the IBSS. This configuration may be realized when STAs communicate directly. Because this type of LAN is often formed without pre-planning for only as long as the LAN is needed, it is often referred to as an ad hoc network.

The membership of an STA in a BSS may be dynamically changed when the STA is powered on or off or the STA moves into or out of the coverage area of the BSS. To be a member of the BSS, an STA may join the BSS by synchronization. To access all services of a BSS infrastructure, the STA should be associated with the BSS. This association may be dynamically performed and may involve use of a Distributed System Service (DSS).

FIG. 6 illustrates another exemplary configuration of the IEEE 802.11 system to which the present invention is applicable. In FIG. 6, components such as a Distribution System (DS), a Distribution System Medium (DSM), and an Access Point (AP) are added to the architecture illustrated in FIG. 5.

Physical (PHY) performance may limit direct STA-to-STA distances. While this distance limitation is sufficient in some cases, communication between STAs apart from each other by a long distance may be required. To support extended coverage, a DS may be deployed.

A DS is built from multiple BSSs that are interconnected. Specifically, a BSS may exist as a component of an extended network with a plurality of BSSs, rather than it exists independently as illustrated in FIG. 5.

The DS is a logical concept and may be specified by the characteristics of a DSM. In this regard, the IEEE 802.11 standard logically distinguishes a Wireless Medium (WM) from a DSM. Each logical medium is used for a different purpose by a different component. The IEEE 802.11 standard does not define that these media should be the same or different. The flexibility of the IEEE 802.11 LAN architecture (DS structure or other network structures) may be explained in the sense that a plurality of media are logically different. That is, the IEEE 802.11 LAN architecture may be built in various manners and may be specified independently of the physical characteristics of each implementation example.

The DS may support mobile devices by providing services needed to handle address to destination mapping and seamless integration of multiple BSSs.

An AP is an entity that enables its associated STAs to access a DS through a WM and that has STA functionality. Data may move between the BSS and the DS through the AP. For example, STA2 and STA3 illustrated in FIG. 2 have STA functionality and provide a function of enabling associated STAs (STA1 and STA4) to access the DS. Since all APs are basically STAs, they are addressable entities. An address used by an AP for communication on the WM is not necessarily identical to an address used by the AP for communication on the DSM.

Data that one of STAs associated with the AP transmits to an STA address of the AP may always be received at an uncontrolled port and processed by an IEEE 802.1X port access entity. If a controlled port is authenticated, transmission data (or frames) may be transmitted to the DS.

FIG. 7 illustrates another exemplary configuration of the IEEE 802.11 system to which the present invention is applicable. In addition to the architecture illustrated in FIG. 6, FIG. 7 conceptually illustrates an Extended Service Set (ESS) to provide extended coverage.

A DS and BSSs allow IEEE 802.11 to create a wireless network of arbitrary size and complexity. IEEE 802.11 refers to this type of network as an ESS network. An ESS may be a set of BSSs connected to a single DS. However, the ESS does not the DS. The ESS network appears as an IBSS network to a Logical Link Control (LLC) layer. STAs within an ESS may communicate with each other and mobile STAs may move from one BSS to another (within the same ESS) transparently to the LLC layer.

IEEE 802.11 assumes nothing about the relative physical locations of the BSSs in FIG. 7. All of the followings are possible. The BSSs may partially overlap. This is commonly used to arrange contiguous coverage. The BSSs may be physically disjointed. Logically, there is no limit to the distance between BSSs. The BSSs may be physically co-located. This may be done to provide redundancy. One (or more) IBSS or ESS networks may be physically present in the same space as one (or more) ESS networks. This may arise when an ad hoc network is operating at a location that also has an ESS network, when physically overlapping IEEE 802.11 networks have been set up by different organizations, or when two or more different access and security policies are needed at the same location.

FIG. 8 illustrates an exemplary configuration of a WLAN system. In FIG. 8, an exemplary infrastructure BSS including a DS is illustrated.

In the example of FIG. 8, an ESS includes BSS1 and BSS2. In the WLAN system, an STA is a device complying with Medium Access Control/Physical (MAC/PHY) regulations of IEEE 802.11. STAs are categorized into AP STAs and non-AP STAs. The non-AP STAs are devices handled directly by users, such as laptop computers and mobile phones. In FIG. 8, STA1, STA3, and STA4 are non-AP STAs, whereas STA2 and STA5 are AP STAs.

In the following description, a non-AP STA may be referred to as a terminal, a Wireless Transmit/Receive Unit (WTRU), a User Equipment (UE), a Mobile Station (MS), a Mobile Terminal (MT), or a Mobile Subscriber Station (MSS). An AP corresponds to a Base Station (BS), a Node B, an evolved Node B (eNB), a Base Transceiver System (BTS), or a femto BS in other wireless communication fields. FIG. 9 is a block diagram of a communication apparatus according to an embodiment of the present invention.

The apparatus shown in FIG. 9 can be a user equipment (UE) and/or eNB adapted to perform the above mechanism, but it can be any apparatus for performing the same operation.

As shown in FIG. 9, the apparatus may comprises a DSP/microprocessor (110) and RF module (transceiver; 135). The DSP/microprocessor (110) is electrically connected with the transceiver (135) and controls it. The apparatus may further include power management module (105), battery (155), display (115), keypad (120), SIM card (125), memory device (130), speaker (145) and input device (150), based on its implementation and designer's choice.

Specifically, FIG. 9 may represent a UE comprising a receiver (135) configured to receive a request message from a network, and a transmitter (135) configured to transmit the transmission or reception timing information to the network. These receiver and the transmitter can constitute the transceiver (135). The UE further comprises a processor (110) connected to the transceiver (135: receiver and transmitter).

Also, FIG. 9 may represent a network apparatus comprising a transmitter (135) configured to transmit a request message to a UE and a receiver (135) configured to receive the transmission or reception timing information from the UE. These transmitter and receiver may constitute the transceiver (135). The network further comprises a processor (110) connected to the transmitter and the receiver. This processor (110) may be configured to calculate latency based on the transmission or reception timing information.

FIG. 10 is a diagram for a LTE-WLAN aggregation (LWA) Radio Protocol Architecture according to embodiments of the present invention.

E-UTRAN supports LTE-WLAN aggregation (LWA) operation whereby a UE in RRC_CONNECTED is configured by the eNB to utilize radio resources of LTE and WLAN. Two scenarios are supported depending on the backhaul connection between LTE and WLAN: i) non-collocated LWA scenario for a non-ideal backhaul and, ii) collocated LWA scenario for an ideal/internal backhaul.

In LWA, the radio protocol architecture that a particular bearer uses depends on the LWA backhaul scenario and how the bearer is set up. Split LWA bearer is depicted on FIG. 10.

The Split LWA bearer is a bearer whose radio protocols are located in both the eNB and the WLAN to use both eNB and WLAN radio resources in LTE-WLAN Aggregation.

The Split LWA bearer comprise a PDCP entity, a RLC entity and a MAC entity for the eNB and a LWAAP (LTE-WLAN Aggregation Adaptation Protocol) entity and a WLAN entity for the WLAN.

An LWAAP entity receives/delivers LWAAP SDUs from/to upper layers (i.e. PDCP) and sends/receives LWAAP PDUs to/from its peer LWAAP entity via WLAN. At the eNB, when an LWAAP entity receives an LWAAP SDU from upper layers, it constructs the corresponding LWAAP PDU and delivers it to lower layers. At the UE, when an LWAAP entity receives an LWAAP PDU from lower layers, it reassembles the corresponding LWAAP SDU and delivers it to upper layers.

For PDUs sent over WLAN in LWA operation, the LWAAP entity generates LWA PDU containing a DRB identity and the WLAN termination (WT) uses the LWA EtherType for forwarding the data to the UE over WLAN. The UE uses the LWA EtherType to determine that the received PDU belongs to an LWA bearer and uses the DRB identity to determine to which LWA bearer the PDU belongs to.

In the downlink, LWA supports split bearer operation where the PDCP sublayer of the UE supports in-sequence delivery of upper layer PDUs based on the reordering procedure introduced for DC. In the uplink, PDCP PDUs can only be sent via the LTE.

The UE supporting LWA may be configured by the eNB to send PDCP status report or LWA PDCP status report, in cases where feedback from WT is not available.

FIG. 11 is an exemplary for a LTE-WLAN aggregation architecture.

The protocol architecture for LTE-WLAN aggregation should be based on Release-12 LTE Dual Connectivity solutions 2C and 3C. FIG. 11 shows architecture option 3C as envisaged. Option 3C resembles the Rel-12 dual connectivity split bearer architecture, where the WLAN termination (WT) takes the role of the secondary eNB.

Using Dual Connectivity 3C-like approaches should allow the WT to provide for bearer specific feedback about: i) packets lost upon transfer from eNB to WT, ii) delivery status as known at the WT (i.e. Wi-Fi MAC ACK/NACK), and iii) flow control data request from WT to eNB according to buffer status at the WT.

FIG. 12 is an example for Dual Connectivity 3C-like approaches using GTP-U.

The termination of the bearer specific GTP-U at the WT, e.g., for transferring flow control information between WT and eNB, may require a new Flow Adaptation Layer to maintain bearer separation of the delivered PDCP packets at the UE.

The Flow Adaptation Layer (FAL) could be located at the eNB as shown in FIGS. 1 and 2 above, but could also be located at the WT: i) FAL at the eNB: This requires a mapping at the WT to link the Xw interface with the air-IF (or eNB with UE, respectively), and ii) FAL at the WT: In that case an additional layer over the air-IF prolongs the bearer specific flows terminated at the WT from Xw side. This requires a mapping at the WT between the Bearer ID used over the air-IF and the GTP-U TEID used over Xw.

By having the FAL in the eNB, the eNB would be in control of the adaptation between PDCP and GTP. In addition, this approach would minimize the impact on the WT since less information needs to be transferred from eNB to WT.

Flow control can be used as a tool (proactive) to determine when the WLAN access cannot meet its share of expected QoS level, and allow LTE to take appropriate actions to ensure that the QoS requirement for EPS bearers are met.

DL flow control was specified in DC to ensure that the transmission buffer in SeNB does not overflow to avoid that more than half the PDCP sequence number space is brought in flight and to ensure that the transmission delay of packets via SeNB is not increased. However, using a similar mechanism to Rel-12 flow control for LTE-WLAN aggregation would require WLAN (i.e. WLAN MAC) to have the ability to provide the necessary flow control information e.g. highest PDCP PDU sequence number successfully delivered in sequence to the UE, Xw-U packets declared as being “lost” by the WLAN, desired buffer size for the concerned E-RAB, minimum desired buffer size in bytes for the UE.

It is unclear if the eNB can solely rely on WLAN access to provide such flow control information in a timely manner as 3GPP is unlikely to enforce strict performance requirement for the WLAN part. Furthermore, it may not be either assumed that the WT has the mean to provide such indication over an interface towards the eNB as such interface may not always be available.

Using PDCP Status reporting does not impose any further requirement over the WiFi MAC. Furthermore, the PDCP Status reporting is already provided as part of the handover and re-establishment procedure and just need to be extended to also cover the case for LTE-WiFi aggregation.

FIG. 13 is a diagram for PDCP control PDU for PDCP status report.

For a transmission operation, when upper layers request a PDCP re-establishment, for radio bearers that are mapped on RLC AM, the UE shall compile a status report as indicated below after processing the PDCP Data PDUs that are received from lower layers due to the re-establishment of the lower layers, and submit it to lower layers as the first PDCP PDU for the transmission if the radio bearer is configured by upper layers to send a PDCP status report in the uplink, by: setting the FMS field to the PDCP SN of the first missing PDCP SDU, if there is at least one out-of-sequence PDCP SDU stored, allocating a Bitmap field of length in bits equal to the number of PDCP SNs from and not including the first missing PDCP SDU up to and including the last out-of-sequence PDCP SDUs, rounded up to the next multiple of 8, or up to and including a PDCP SDU for which the resulting PDCP Control PDU size is equal to 8188 bytes, whichever comes first, setting as ‘0’ in the corresponding position in the bitmap field for all PDCP SDUs that have not been received as indicated by lower layers, and optionally PDCP SDUs for which decompression have failed, and indicating in the bitmap field as ‘1’ for all other PDCP SDUs.

For a receive operation, when a PDCP status report is received in the downlink, for radio bearers that are mapped on RLC AM: for each PDCP SDU, if any, with the bit in the bitmap set to ‘1’, or with the associated COUNT value less than the COUNT value of the PDCP SDU identified by the FMS field, the successful delivery of the corresponding PDCP SDU is confirmed, and the UE shall process the PDCP SDU.

The PDCP Control PDU is used to convey: i) a PDCP status report indicating which PDCP SDUs are missing and which are not following a PDCP re-establishment, and ii) header compression control information, e.g. interspersed ROHC feedback.

FIG. 13A shows a format of the PDCP Control PDU carrying one PDCP status report when a 12 bit SN length is used, FIG. 13B shows the format of the PDCP Control PDU carrying one PDCP status report when a 15 bit SN length is used, and FIG. 13C shows the format of the PDCP Control PDU carrying one PDCP status report when an 18 bit SN length is used. This format is applicable for DRBs mapped on RLC AM.

A D/C field indicates that whether the PDCP PDU is for a control PDU or a Data PDU. If a value of the D/C field is set to ‘0’, the PDCP PDU is for Control PDU. Otherwise, the PDCP PDU is for a Data PDU.

A PDU type field indicates whether the PDCP control PDU is for PDCP status report or for Interspersed ROHC feedback packet. If a value of the PDU type field is set to ‘000’, the PDCP control PDU is for PDCP status report. If a value of the PDU type field is set to ‘001’, the PDCP control PDU is for Interspersed ROHC feedback packet. A FMS field indicates a PDCP SN of the first missing PDCP SDU. A size of the FMS field is 12-bits when a 12 bit SN length is used, a size of the FMS field is 15-bits when a 15 bit SN length is used, and a size of the FMS field is 18-bits when an 18 bit SN length is used.

A most significant bit (MSB) of the first octet of the type “Bitmap” indicates whether or not the PDCP SDU with the SN (FMS+1) modulo (Maximum_PDCP_SN+1) has been received and, optionally decompressed correctly. A least significant bit (LSB) of the first octet of the type “Bitmap” indicates whether or not the PDCP SDU with the SN (FMS+8) modulo (Maximum_PDCP_SN+1) has been received and, optionally decompressed correctly. If a bit of the bitmap indicates ‘0’, PDCP SDU with PDCP SN=(FMS+bit position) modulo (Maximum_PDCP_SN+1) is missing in the receiver. The bit position of Nth bit in the Bitmap is N, i.e., the bit position of the first bit in the Bitmap is 1.

If a bit of the bitmap indicates ‘1’, PDCP SDU with PDCP SN=(FMS+bit position) modulo (Maximum_PDCP_SN+1) does not need to be retransmitted. The bit position of Nth bit in the Bitmap is N, i.e., the bit position of the first bit in the Bitmap is 1.

The UE fills the bitmap indicating which SDUs are missing (unset bit-‘0’), i.e. whether an SDU has not been received or optionally has been received but has not been decompressed correctly, and which SDUs do not need retransmission (set bit-′1′), i.e. whether an SDU has been received correctly and may or may not have been decompressed correctly.

As mentioned before, The LTE-WLAN Aggregation (LWA) is designed based on the Dual Connectivity (DC), where the UE PDCP receives PDCP PDUs from two path, one from LTE and the other from WLAN.

One important function for the PDCP is flow control. The PDCP transmitter must control the number of PDCP PDUs being transmitted less than the half of the PDCP SN space so that the successfully transmitted PDCP PDUs are not discarded by the PDCP receiver. When the last delivered PDCP SDU to upper layer in the PDCP receiver is SN=X, if the PDCP receiver receives PDCP SDU with SN>=X+Window (=half of PDCP SN space), the PDCP receiver discards the received PDCP SDU because it is outside of the Window.

However, since the WLAN side is not under control of eNB, the flow control between eNB and WLAN AP is difficult to realize. Thus, some of the prior arts suggest for the UE PDCP to send PDCP status report periodically to the eNB PDCP. The PDCP status report consists of First Missing PDCP SN (FMS) and BITMAP which represents reception status of each PDCP PDU following PDCP PDU with SN=FMS.

However, periodical transmission of PDCP status report is not radio-efficient. Most of the information is of no use for flow control, and periodical transmission of useless information on the radio is just waste of radio resource.

FIG. 14 is a diagram for transmitting Flow Control Information for a LTE-WLAN aggregation (LWA) according to embodiments of the present invention.

To reduce the signaling overhead caused by frequent transmission of PDCP status report while providing necessary information for the flow control in a timely manner, it is invented that the PDCP receiver transmits a “Flow Control Information (FCI)” to the PDCP transmitter.

The PDCP receiver checks whether one of the FCI transmission conditions is met or not for each TTI or for each PDCP SDU reception (S1401).

The FCI transmission condition comprises i) when the amount of PDCP SDUs (or PDUs) stored in the PDCP reordering buffer exceeds a threshold (condition 1), ii) when the percentage of [amount of PDCP SDUs stored in the PDCP reordering buffer/PDCP reordering buffer size] exceeds a threshold (condition 2), iii) when the number of PDCP SDUs (or PDUs) stored in the PDCP reordering buffer exceeds a threshold (condition 3), iv) when the SN gap between the PDCP SN of next expected PDCP SDU and the PDCP SN of last delivered PDCP SDU exceeds a threshold, i.e. Next_PDCP_RX_SN-Last_Submitted_PDCP_RX_SN exceeds a threshold (condition 4), or v) when the PDCP reordering timer expires, if the SN gap between the PDCP SN of an updated last delivered PDCP SN and the old last delivered PDCP SN exceeds a threshold, i.e. updated Last_Submitted_PDCP_RX_SN-old Last_Submitted_PDCP_RX_SN exceeds a threshold (condition 5).

The Next_PDCP_RX_SN indicates the next expected PDCP SN by the receiver for a given PDCP entity. At establishment of the PDCP entity, the UE shall set Next_PDCP_RX_SN to 0. The Last_Submitted_PDCP_RX_SN indicates the SN of the last PDCP SDU delivered to the upper layers. At establishment of the PDCP entity, the UE shall set Last Submitted PDCP_RX_SN to Maximum_PDCP_SN.

And the Maximum_PDCP_SN can be 65535 if the PDCP entity is configured for the use of 16 bits SNs, or 32767 if the PDCP entity is configured for the use of 15 bits SNs, or 4095 if the PDCP entity is configured for the use of 12 bit SNs, or 127 if the PDCP entity is configured for the use of 7 bit SNs, or 31 if the PDCP entity is configured for the use of 5 bit SNs.

Preferably, the threshold of each case is configured by the eNB through RRC message. It is configured per RB or per UE.

And if the one of the FCI transmission conditions is met, the PDCP receiver can transmit Flow Control Information (FCI) to a PDCP transmitter (S1403).

Preferably, the FCI includes one or more of followings: i) the amount of PDCP SDUs stored in the PDCP reordering buffer, ii) the percentage of [amount of PDCP SDUs stored in the PDCP reordering buffer/PDCP reordering buffer size], iii) the number of PDCP SDUs (or PDUs) stored in the PDCP reordering buffer, iv) Last_Submitted_PDCP_RX_SN at the time of transmission of FCI, v) Next_PDCP_RX_SN at the time of transmission of FCI, vi) PDCP SN of first missing PDCP SDU, vii) the highest PDCP SN among received PDCP SDUs, viii) the highest PDCP SN among stored PDCP SDUs, ix) PDCP SNs of each missing PDCP SDU, or x) PDCP SNs of each PDCP SDU stored in the PDCP reordering buffer.

Preferably, the FCI is transmitted via PDCP Control PDU, RLC Control PDU, MAC Control Element, or RRC message.

Preferably, the FCI may be PDCP status report or LWA status report.

When LWA status report is triggered, the UE shall compile a status report as indicated below, and submit it to lower layers as the first PDCP PDU for the transmission, by i) setting the FMS field to the PDCP SN of the first missing PDCP SDU, ii) setting the HRW field to the PDCP SN of the PDCP SDU received on WLAN with highest PDCP COUNT value or to FMS if no PDCP SDUs have been received on WLAN, and iii) setting the NMP field to the number of missing PDCP SNs.

Preferably, the PDCP receiver checks, for each TTI or for each PDCP SDU reception, whether the FCI transmission condition is met.

After a FCI is transmitted, the PDCP receiver may avoid transmission of another FCI for a time period even if the FCI transmission condition is met. The avoidance of transmission of another FCI is controlled by a FCI prohibit timer. The PDCP receiver starts the FCI prohibit timer when a FCI is transmitted (S1405), and while the timer is running, the PDCP receiver does not check the FCI transmission condition or does not transmit another FCI even if the condition is met (S1407).

If the FCI transmission condition is met while the timer is running, the PDCP receiver transmits a FCI only after the FCI prohibit timer expires (S1409).

FIG. 15 is an example for transmitting Flow Control Information for a LTE-WLAN aggregation (LWA) according to embodiments of the present invention.

Let's take an example. Assume that following thresholds are configured for each condition: i) a threshold of the condition 1 is 1000 bytes, ii) a threshold of the condition 2 is 70%, iii) a condition of the condition 3 is 2, iv) a threshold of the condition 4 is 3, and v) a threshold of the condition 5 is 2. and assume the PDCP reordering buffer size is 2000-bytes.

Assume the following situation.

1. at t=T0, Last_Submitted_PDCP_RX_SN=11 and Next_PDCP_RX_SN=12.

In this case, the amount of PDCP SDUs stored in the PDCP reordering buffer is 0 bytes, the number of PDCP SDUs stored in the PDCP reordering buffer is 0, and Next_PDCP_RX_SN-Last_Submitted_PDCP_RX_SN equals 1.

2. at t=T1, the PDCP receiver receives a PDCP SDU of SDU13=300 bytes. The PDCP receiver stores the SDU13 in the PDCP reordering buffer. The PDCP receiver updates Next_PDCP_RX_SN to 14. The PDCP receiver starts PDCP reordering timer for SDU14.

In this case, the amount of PDCP SDUs stored in the PDCP reordering buffer is 300 bytes, the number of PDCP SDUs stored in the PDCP reordering buffer is 1, and Next_PDCP_RX_SN-Last_Submitted_PDCP_RX_SN equals 3.

3. at t=T2, the PDCP receiver receives a PDCP SDU of SDU14=800 bytes. The PDCP receiver stores the SDU14 in the PDCP reordering buffer. The PDCP receiver updates Next_PDCP_RX_SN to 15.

In this case, the amount of PDCP SDUs stored in the PDCP reordering buffer is 1100 bytes, the number of PDCP SDUs stored in the PDCP reordering buffer is 2, and Next_PDCP_RX_SN-Last Submitted PDCP_RX_SN equals 4.

4. at t=T3, the PDCP receiver receives a PDCP SDU of SDU17=500 bytes. The PDCP receiver stores the SDU17 in the PDCP reordering buffer. The PDCP receiver updates Next_PDCP_RX_SN to 18.

In this case, the amount of PDCP SDUs stored in the PDCP reordering buffer is 1100 bytes, the number of PDCP SDUs stored in the PDCP reordering buffer is 2, and Next_PDCP_RX_SN-Last_Submitted_PDCP_RX_SN equals 4.

5. at t=T4, the PDCP reordering timer expires. The PDCP receiver delivers SDU13 and SDU14 to upper layer. The PDCP receiver updates Last_Submitted_PDCP_RX_SN to 14. The PDCP receiver starts PDCP reordering timer for SDU18.

In this case, the amount of PDCP SDUs stored in the PDCP reordering buffer is 200 bytes, because, the SDU 13 with 300 bytes and the SDU14 with 800 bytes are submitted and only the SDU 17 with 500 bytes is stored in the PDCP reordering buffer. Thus, the number of PDCP SDUs stored in the PDCP reordering buffer is 1, and Next_PDCP_RX_SN-Last_Submitted_PDCP_RX_SN equals 4, and new Last_Submitted_PDCP_RX_SN-old Last_Submitted_PDCP_RX_SN equals 3.

Then, the PDCP receiver transmits the FCI to the PDCP transmitter at following time point under each of the conditions, mentioned above.

Under the condition 1 (=when the amount of PDCP SDUs (or PDUs) stored in the PDCP reordering buffer exceeds a threshold), because the amount of PDCP SDUs stored in the PDCP reordering buffer which is 1100 bytes exceeds 1000 bytes at t=T2, the PDCP receiver transmits the FCI to the PDCP transmitter at t=T2.

Under the condition 2 (=when the percentage of [amount of PDCP SDUs stored in the PDCP reordering buffer/PDCP reordering buffer size] exceeds a threshold), because the percentage of [amount of PDCP SDUs stored in the PDCP reordering buffer/PDCP reordering buffer size] which is 80% exceeds 70% at t=T3, the PDCP receiver transmits the FCI to the PDCP transmitter at t=T3.

Under the condition 3 (=when the number of PDCP SDUs (or PDUs) stored in the PDCP reordering buffer exceeds a threshold), because the number of PDCP SDUs stored in the PDCP reordering buffer which is 3 exceeds 2 at t=T3, the PDCP receiver transmits the FCI to the PDCP transmitter at t=T3.

Under the condition 4 (=when the SN gap between the PDCP SN of next expected PDCP SDU and the PDCP SN of last delivered PDCP SDU exceeds a threshold, i.e. Next_PDCP_RX_SN-Last_Submitted_PDCP_RX_SN exceeds a threshold), because Next_PDCP_RX_SN-Last_Submitted_PDCP_RX_SN which is 4 exceeds 3 at t=T2, the PDCP receiver transmits the FCI to the PDCP transmitter at t=T2.

Lastly, under the condition 5 (=when the PDCP reordering timer expires, if the SN gap between the PDCP SN of an updated last delivered PDCP SN and the old last delivered PDCP SN exceeds a threshold), because the PDCP reordering timer expires and updated Last_Submitted_PDCP_RX_SN(=14)-old Last_Submitted_PDCP_RX_SN(=11) which is 3 exceeds 2 at t=T4, the PDCP receiver transmits the FCI to the PDCP transmitter at t=T4.

The embodiments of the present invention described hereinbelow are combinations of elements and features of the present invention. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present invention may be constructed by combining parts of the elements and/or features. Operation orders described in embodiments of the present invention may be rearranged. Some constructions of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions of another embodiment. It is obvious to those skilled in the art that claims that are not explicitly cited in each other in the appended claims may be presented in combination as an embodiment of the present invention or included as a new claim by subsequent amendment after the application is filed.

In the embodiments of the present invention, a specific operation described as performed by the BS may be performed by an upper node of the BS. Namely, it is apparent that, in a network comprised of a plurality of network nodes including a BS, various operations performed for communication with an MS may be performed by the BS, or network nodes other than the BS. The term ‘eNB’ may be replaced with the term ‘fixed station’, ‘Node B’, ‘Base Station (BS)’, ‘access point’, etc.

The above-described embodiments may be implemented by various means, for example, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments of the present invention may be implemented by one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to the embodiments of the present invention may be implemented in the form of modules, procedures, functions, etc. performing the above-described functions or operations. Software code may be stored in a memory unit and executed by a processor. The memory unit may be located at the interior or exterior of the processor and may transmit and receive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present invention may be carried out in other specific ways than those set forth herein without departing from essential characteristics of the present invention. The above embodiments are therefore to be construed in all aspects as illustrative and not restrictive. The scope of the invention should be determined by the appended claims, not by the above description, and all changes coming within the meaning of the appended claims are intended to be embraced therein.

INDUSTRIAL APPLICABILITY

While the above-described method has been described centering on an example applied to the 3GPP LTE system, the present invention is applicable to a variety of wireless communication systems in addition to the 3GPP LTE system. 

1-10. (canceled)
 11. A method for a user equipment (UE) operating in a wireless communication system, the method comprising: checking, by a Packet Data Convergence Protocol (PDCP) entity, whether a FCI transmission condition is met or not; and transmitting Flow Control Information (FCI) when the FCI transmission condition is met, wherein the FCI includes a PDCP Sequence Number (SN) of a first missing PDCP Service Data Unit (SDU) and a highest PDCP SN among received PDCP SDUs.
 12. The method according to claim 11, wherein the FCI is transmitted through status report.
 13. The method according to claim 11, wherein the PDCP entity is associated with a bearer whose radio protocols are located in both a e-NodeB (eNB) and Wire-less LAN (WLAN) to use both eNB and WLAN radio resources in LTE-WLAN Aggregation.
 14. The method according to claim 11, further comprising: starting a FCI prohibit timer when the FCI is transmitted; and transmitting another FCI only after the FCI prohibit timer expires, wherein the UE doesn't check the FCI transmission condition is met, and transmit the FCI, while the FCI prohibit timer is running.
 15. A method for a user equipment (UE) operating in a wireless communication system, the method comprising: checking, by a Packet Data Convergence Protocol (PDCP) entity, whether an amount of PDCP Service Data Units (SDUs) stored in a PDCP reordering buffer is above a first threshold for each transmission time interval (TTI) or for each PDCP SDU reception; and transmitting Flow Control Information (FCI) when the amount of PDCP SDUs stored in the PDCP reordering buffer is above the first threshold.
 16. The method according to claim 15, wherein when the UE checks whether the amount of PDCP SDUs stored in the PDCP reordering buffer is above the first threshold, a value of percentage of the amount of PDCP SDUs stored in the PDCP reordering buffer from a size of the PDCP reordering buffer is used.
 17. The method according to claim 15, further comprising: checking, by the PDCP entity, whether a number of PDCP SDUs stored in the PDCP reordering buffer is above a second threshold for each TTI or for each PDCP SDU reception; and transmitting FCI when the number of PDCP SDUs stored in the PDCP reordering buffer is above the second threshold.
 18. The method according to claim 17, wherein when the UE checks whether the number of PDCP SDUs stored in the PDCP reordering buffer is above the second threshold, a sequence number (SN) gap between a PDCP SN of next expected PDCP SDU and a PDCP SN of last delivered PDCP SDU is used.
 19. The method according to claim 15, further comprising: checking, by the PDCP entity, whether a number of PDCP SDUs delivered to an upper layer is above a third threshold for each TTI or for each PDCP SDU reception at a reordering timer expires; and transmitting FCI when the number of PDCP SDUs delivered to the upper layer is above the third threshold.
 20. The method according to claim 19, wherein when the UE checks the number of PDCP SDUs delivered to the upper layer is above the third threshold, a sequence number (SN) gap between a PDCP SN of an updated last delivered PDCP SN and an old last delivered PDCP SN is used.
 21. A user equipment (UE) operating in a wireless communication system, the UE comprising: a Radio Frequency (RF) module; and a processor operably coupled with the RF module and configured to: check whether an amount of PDCP Service Data Units (SDUs) stored in a PDCP reordering buffer is above a first threshold for each transmission time interval (TTI) or for each PDCP SDU reception; and transmit Flow Control Information (FCI) when the amount of PDCP SDUs stored in the PDCP reordering buffer is above the first threshold. 